1. Field of the Invention
The present invention is related to an overdrive method for anti-double edge of LCD, and especially to an overdrive method which can effectively eliminate double edge phenomenon generated during overdrive liquid-crystal image developing by selection from an ADE (anti-double edge) Lookup Table (LUT) containing therein driving-voltage values of a present frame and a previous frame, overdrive voltage values and actually achieved voltage values. By adding a complementary table and an operation unit, the method can eliminate double edge phenomenon generated because of different response speeds; and thereby is suitable for various liquid crystal displays.
2. Description of the Prior Art
The liquid crystal display device allows obtaining a highly precise display. However, since the LCD does not have a sufficient image quality in displaying a moving pictures, so that there is an index-response time to discriminate being rood or bad of the performance of a liquid crystal display. Generally, liquid crystal displays are divided into two kinds when being not added with voltage: Normally White (NW) mode, and Normally Black (NB) mode; wherein the Normally White mode means that a display panel has a transparent frame when being not added with voltage, that is a bright frame; the Normally Black mode means that a display panel has an obscure state when being not added with voltage, that is a black frame. Taking the Normally White mode as an example, the response time is divided into two parts:    (1) ascending response time: this is the twist time required for liquid crystal to make the brightness of a liquid crystal box of a liquid crystal display to change from 90% to 10%, and is called “Tr” under adding with voltage; and    (2) descending response time: this is the restoring time required for the liquid crystal to make the brightness of the liquid crystal to change from 10% to 90%, and is called “Tf” when being not added with voltage.
Generally, when the speed of developing of pictures exceeds 25 pieces/sec., the eyes of a person will take the pictures changing fast as continuous pictures; while in modern family amusements, such as in playing a high-quality DVD movie, the speed of developing the fast moving pictures normally is larger than 60 frames/sec. In other words, the time interval of each frame is 1/60=16.67 ms; if the response time of a liquid crystal display is larger than the frame interval, it will induce traces of residual images or tabs to seriously affect the qualities of images observed. It should be viewed from the factors affecting the response time to know how to increase the speed of response. The following equations are respectively the calculating equations for the ascending response time Tr and the descending response time Tf:
      T    r    =                                          γ            1                    ⁢                      d            2                                    Δɛ          ⁡                      (                                          V                2                            -                              V                th                2                                      )                              ⁢                          ⁢              T        f              =                            γ          1                ⁢                  d          2                                      Δɛ          ⁢                                          ⁢                      V            th            2                          )            
Wherein γ1: the viscosity coefficient of the liquid crystal;
V: the driving voltage of the liquid crystal box;
Δ∈: the dielectric coefficient of the liquid crystal.
It can be known from the above statement that there are four ways to reduce the response time of the liquid crystal display: to lower the viscosity coefficient of the liquid crystal, to reduce gaps of the liquid crystal box, to increase the driving voltage and to increase the dielectric coefficient, wherein the technique to increase the driving voltage is called an “overdrive” technique frequently applied in the LCD industry, an increased voltage higher than original gray level voltage can be transferred by a liquid-crystal driver IC to a liquid crystal panel to increase the twist voltage of the liquid crystal for increasing response speeds of liquid crystal and improving a motion blur of LCD panel, and thereby the liquid crystal can twist and restore faster to rapidly get the brightness of the image data to be presented. Some conventional methods and devices for improving the display characteristics of a liquid crystal display panel aim at improvement in the response speed of liquid crystal to reach a predetermined transmission rate in a single frame period, such as ones described in U.S. Pat. Nos. 6,825,821 and 7,148,869.
U.S. Pat. No. 6,825,821 describes an over-driving method using a brief table to store the over-driving image data. The brief table only includes part of the over-driving image data for driving the pixels switched from one gray scale to another. When the driving circuit receives the image data from the input terminal, a processor is used to perform an interpolation operation to expand the brief table. Hence, an extra algorithm is needed in the conventional over-driving method.
The values of overdrive voltage are obtained by measuring optical reaction curves of pixels of a LCD panel switched from any gray scale value to other gray scale values within a frame period and generating a standard overdrive look-up table (OD-LUT) according to the reaction curves measured. As shown in FIG. 7, it is a chart to indicate how the values of OD-LUT with the OD target time are selected. In the conventional method, an OD target time is measured by finding an intersection point of OD target time and the 100% luminance of desired target code. Further, a driving code that takes response of liquid crystal to the target level after just 1 frame of 16.7 ms is searched; that is, the response curve driven by a desired overdrive voltage mostly close to the intersection point around the OD target time is searched. For example, if OD target time is 16.7 ms and the driving code is from 16 to 192, we can find the intersection point of the 16.7 ms line and the 100% luminance of driving code 192. FIG. 7 shows that the curve of driving code from 16 to 232 is close to the intersection point mostly. And the value of the OD-LUT is 232 for driving code from 16 to 192, and the overdrive voltage of driving code from 16 to 232 is determined for driving the driving code from 16 to 192. In cases of image data of 6 bits (64 gray-scale), the OD-LUT provided in accordance with the number of gray-scale of image data stores 4096 pieces of data for the combinations of 64 gray-scale of a current frame and 64 gray-scale of a previous frame. Such that the physical characteristic of liquid crystal response between two gray levels is obtained.
Up to now, the OD-LUT is usually code-by-code adjusted/produced manually with operator's gaze by eyes to find out the results equivalent to the human visual perception, or produced by an automatic system to recursively search the optimal OD-LUT according to the MPRT (moving picture response time) of LCD, such as a system having a pursuit/tracking camera, high speed/fixed camera or a fixed optical detectors. In cases of the size of desired OD-LUT is 6×6, the operator will manually adjust the overdrive value of LUT one by one. By observing the motion of desired patterns, the operator chooses the appropriate value which not only improves the dynamic image quality with least motion blur but also makes the moving edge with least side-effect. When the overdrive voltage value of a conventional overdrive technique is getting close to a maximum value (code 255) and a minimum value (code 0), it is unable to render a liquid crystal to twist smoothly within the time interval of a frame to get an object value; as is shown in FIG. 8, an overdrive voltage value is added within the time interval of a first frame in order to get an object value of the driving voltage of a present frame, however the actually achieved voltage value is inferior; then another overdrive voltage value is added within the time interval of a second frame in order to get an object value of the driving voltage of a present frame, but rather, the error of the first frame renders the actually achieved voltage value to exceed the object value. Such error renders the liquid crystal panel to get a situation of being much brighter or darker than the bottom color during displaying of the liquid crystal panel; such situation is called “Double Edge”. However, even thought the overdrive is a well known method and be widely used in LCD-TV, there is still no efficient and systematic way to find out the optimal OD-LUT which can solve both the problems of double edges and motion blur for various LCD panel. In order to get rid of the phenomenon of Double Edge created by the nature of liquid crystal in the conventional overdrive technique, big manufacturers in the art has proposed several solving countermeasures presently. However, the solving measures known presently all use a lookup table having overdrive codes (voltage values) and a lookup table having actually achieved voltage values of liquid crystal to respectively execute access, and then to make comparison of frame data of a previous frame temporarily stored in a frame register with frame data of a present frame coming from an input source, thus an overdrive code can be obtained and output.
By virtue that these operations all need the two lookup tables, while a lookup table also means that it needs two times of resource space for a memory, this forms an extremely heavy burden for a display with an extremely limited resource space for the memory. For example, LCDs for television and PC monitor having 8-bit gray levels may need tables for three colors of RGB and polarity of even/odd respectively, and it needs 12 tables with 212 entries. In the other hand, it is the common tendency in the art of liquid crystal panels to increase speeds of response; while at such faster speeds of response, corresponding contrast data are required for their double edge phenomenon.
In view of the above conventional defects to be solved for providing a new countermeasure to solve the phenomenon of Double Edge for singular response speeds and different response speeds only with an anti-double edge lookup table (ADE LUT), the inventor provides the present invention based on his practical professional experience of industry in academic studying, designing and improvements.